Melting Glaciers Imperil Some--But Not All--Asian Rivers

Melting glaciers in Asia could cause food shortages for up to 60 million people who live in the region's major river basins, a new study finds.

But the research, published yesterday in Science, found that the shrinking glaciers will have less of an impact on Asia's freshwater supply than estimated in the last report of the Intergovernmental Panel on Climate Change.

That 2007 report suggested loss of glaciers and snowpack could eventually leave "hundreds of millions" of people in Asia without sufficient water. It has also come under fire for an error-riddled paragraph that claimed Himalayan glaciers could disappear by 2035.

But the new study by researchers in the Netherlands suggests that increased rainfall in some river basins will blunt the effect of the disappearing snow and ice.

The scientists, led by Walter Immerzeel of FutureWater and Utrecht University, examined how climate change will affect five major Asian rivers -- often referred to as the region's "water towers" -- that together supply water to more than 1.4 billion people, roughly one-fifth of the world's population.

They include the Indus, which begins in the Tibetan Plateau and runs through Pakistan; the Ganges, which traverses India and Bangladesh; the Brahmaputra, which winds through the Himalayas into India; the Yangtze and Yellow rivers, which run through China.

Previous studies, including the IPCC report, had suggested that global warming could accelerate the melting of mountain snowpack and glaciers that feed the river systems, and lead to major water shortages by the end of this century.

But such predictions have been hard to quantify, the new study notes, because of limited data.

Some basins to suffer more than others
The Dutch research team tackled the problem by examining the role glacial meltwater plays in each river basin, compared with other sources of freshwater -- rainfall and melting snowpack.

They found that the role of meltwater varied widely from basin to basin.

"We show that meltwater is extremely important in the Indus basin and important for the Brahmaputra basin," they wrote, "but plays only a modest role for the Ganges, Yangtze and Yellow rivers."

In the Indus basin, for example, they conclude that meltwater supplies nearly one and a half times more water than does rainfall downstream. But in the Brahmaputra basin, meltwater contributes a quarter of the water supplied by downstream rains.

Given those differences, the scientists said, it's also clear that some basins would suffer more than others in coming decades if climate change continues at its current pace.

According to their study, warming will eventually lessen the amount of meltwater that flows into each river from glaciers, and it will affect patterns of rain and snowfall -- both the amount of precipitation and its seasonal timing.

"The effects in the Indus and Brahmaputra basins are likely to be severe," they write, "owing to the large population and the high dependence on irrigated agriculture and meltwater."

The scientists estimate that between 28 million and 41 million people in the Brahmaputra river basin could face more trouble securing the food they need. But at the opposite end of the spectrum is the Yellow River, the only of five basins the study suggests could support a higher population thanks to future climate change.

It predicts that by 2050, increased rainfall would allow between 2.4 million and 3.6 million more people in the river basin will be able to secure food.